4 point cages for drag racing have different requirements than road racing. Drag will be cheaper. I don't think drag bars require a diagonal bar.
The reason some track organizations allow the s2000, boxter, and I think the newer miatias with OEM roll hoops is because they are built from the ground up as convertibles. Meaning they have other structural reinforcement from the factory in those examples.
I can speak of the s2000 from personal experience. The roll "hoops" are
not just simple 2 point hoops over the back seat. They are bolted to the chassis of the car with diagonal reinforcement. Also the "A" pillar of that car is designed to support the weight of the car if upside down.
BMW convertibles have none of that. To my knowledge.
As I mentioned in your other thread some shops offer a 4 point cage for the 2 series coupe (Strom Motorsports). I'm sure they use a jig to build the cage. I highly doubt the same jig will work for a convertible. You will need to go to shop and leave the car with them to build one specific for the convertible. Cage for my 370z ~$2k for a 4 point with diagonal bar and harness loops. (if i remember right).
A one off cage for you can be twice that ++.
If you get a 4 point cage then you loose the back seat; so why not just get a two seat convertible with factory hoops like a boxter or Z4 that some organizations will allow?
Or pay up and get a custom cage for the 2 series....or stick to autocross.
Pontiac Solstice
S2000
